This nonprovisional application claims priority under 35 U.S.C. xc2xa7 119(a) on patent application Ser. No. 091121426 filed in TAIWAN on Sep. 19, 2002, which is herein incorporated by reference.
1. Field of the Invention
The invention relates to a pixel structure, and more particularly to a current programmed pixel structure for an active matrix organic light emitting diode.
2. Description of the Related Art
Generally, in an active matrix display, images are displayed by numerous pixels in the matrix, and brightness of each pixel is controlled according to brightness data.
FIG. 1 show a conventional pixel structure 10 for an active matrix organic light emitting diode (AMOLED). The transistor T1 is turned on when the scan line is activated in the programming state, and the data line sinks or supplies current for the specific driving transistor T2. Meanwhile, gate-source voltage of the transistor T2 is adjusted and stored in the storage capacitor C1. In the next state while the scan line is deactivated, often called the reproduction state, the transistor T1 is turned off and the transistor T2 is electrically separated from the data line. The gate-source voltage stored in the storage capacitor C1 may reproduce the current for the OLED, which illuminates accordingly. Threshold voltage of each driving transistor T2 in the conventional pixel structure, however, deviates due to process variation, and this deviation may result in great variation of the output driving current through OLEDs, such that the brightness of each OLED is discordant and there is lack of uniformity in the OLEDs.
Therefore, the improved pixel structure 20 shown in FIG. 2 is promoted. Transistors T3 and T4 are turned on when the scan lines SCAN1 and SCAN2 are activated in the programming state, and the data line sinks or supplies current through the transistor T5, such that the driving current may flow through the OLED and the storage capacitor C2 is charged or discharged due to the current mirror structure composed of transistors T5 and T6. In the reproduction state, the transistors T3 and T4 are turned off when the scan lines SCAN1 and SCAN2 are deactivated, such that the transistor T6 is electrically separated from the data line, and the gate-source voltage of the transistor T5 is stored by the storage capacitor C2. Based on this structure, the current through transistor T5 is                     I        OLED                    I        5              =                  (                              W            6                    xc3x97                      L            5                          )                    (                              W            5                    xc3x97                      L            6                          )              ,
Therefore       I    OLED    =            I      5        xc3x97                  (                              W            6                    xc3x97                      L            5                          )                    (                              W            5                    xc3x97                      L            6                          )            
and then the driving current flowing into the OLED is                                           I            5                    =                                                    k                ⁡                                  (                                      Vgs                    -                    Vt                                    )                                            2                        xc3x97                                          W                5                                            L                5                                                    ,        and                                                      I            OLED                    =                                                    k                ⁡                                  (                                      Vgs                    -                    Vt                                    )                                            2                        xc3x97                                          W                6                                            L                6                                                    ,                              wherein            ⁢                          xe2x80x83                        ⁢            k                    =                                    μ              ⁢                              xe2x80x83                            ⁢              Cox                        2                              
Thus, the driving current flows into the OLED according to sizes of the transistors T5 and T6, and regardless of threshold voltage and process variation of the transistors.
In the current programming pixel structure 20, the voltage on the drain terminal of the transistor T5, however, is increased to VDD when the scan line is deactivated, such that this voltage of the transistor T5 is coupled to the storage capacitor C2 by the parasitical capacitor between the gate terminal and drain terminal. Therefore, this deviation may still result in variation of the output driving current through OLEDs.
The present invention is directed to a current programmed AMOLED pixel structure capable of providing current to OLEDs stably and precisely, regardless of process variation.
The present invention is also directed to a current programmed AMOLED pixel structure capable of improving switching effect caused by switching transistors, thereby increasing reliability.
In the present invention, a first switching transistor has a control terminal coupled to a first scan line, and a first terminal coupled to a data line. A first P-type transistor has a drain terminal and a gate terminal coupled to each other, and a source terminal coupled to a voltage source, and the drain terminal is also coupled to a second terminal of the first switching transistor. A second switching transistor has a first terminal coupled to the gate terminal of the first P-type transistor, and a control terminal coupled to a second scan line. A second P-type transistor has a source terminal coupled to the voltage source, and a gate terminal coupled to a second terminal of the second switching transistor. A storage capacitor is coupled between the voltage source and the gate terminal of the second P-type transistor. An OLED has an anode coupled to the drain terminal of the second P-type transistor and a cathode coupled to ground.